1. Field of the Invention
This invention relates to a rotor for an electric motor which comprises a frame and magnets provided on the frame and a method of making the rotor.
2. Description of the Related Art
FIG. 13 illustrates a three-phase brushless DC motor of the outer rotor type in which a rotor is disposed outside a stator. The motor is designated by reference numeral 101. The motor 101 comprises a stator 102 and a rotor 103. The stator 102 includes a stator core 104 formed by laminating a number of silicon steel plates. The stator core 104 includes an annular yoke 105 and a number of teeth 106 protruding outward from the yoke 105. An insulating resin end plate 107 made of a synthetic resin is mounted on an outer face of the stator core 104. The end plate 107 includes coil winding portions 108 located so as to cover the teeth 106. Coils 109 are wound on the coil winding portions 108. Thus, the coil winding portions 108 constitute an insulation layer between the stator core 104 and the coils 109.
A plurality of mounting portions 112 having mounting holes 112a are formed in an inner circumferential side of the stator core 104. Only one of the mounting holes 112a is shown. A bolt 113 is inserted through each mounting hole 112a, and a distal end of each bolt 113 is further inserted through a hole (not shown) of a stator fixing portion 114. A nut 113a is engaged with each bolt 113 so that the stator 102 is fixedly mounted on the stator fixing portion 114 further fixed to the mounting plate 115.
On the other hand, the rotor 103 is disposed outside the stator 102 so as to cover the latter radially outward. The rotor 103 includes a generally cup-shaped frame 1 made of a synthetic resin. A rotational shaft 118 is fixed to a boss 117 further fixed to a central bottom of the frame 1. The rotational shaft 118 is rotatably supported on bearings 121 and 122 provided on the stator fixing portion 114 and the mounting plate 115 respectively.
Magnets 2 made by sintering ferrite are disposed on an inner circumferential portion of the frame 1 of the rotor 103. The number of the magnets 2 is equal to the number of magnetic poles. A resin 3 is provided on the outer and inner circumferential portions of the frame 1 to integrate the frame 1 and the magnets 2. Each magnet 2 has an inner face opposed to the distal end face of each tooth 106 with a predetermined gap therebetween.
The resin 3 is used only to integrate the frame 1 and the magnets 2 in the above-described rotor 103 but not useful for improving characteristics of the motor. Nonetheless, the resin 3 occupies a large space and accordingly increases the size, weight and costs of the rotor 103 and accordingly, of the motor. Furthermore, in forming the rotor 103, the frame 1 is placed in a molding die (not shown) used to form the resin 3, and the sintered magnets 2 are disposed at the inner circumferential side of the frame 1. However, molding steps are complicated and reduce a manufacturing efficiency.
Further, final positions of the magnets 2 depend upon the resin 3 which is thereafter poured into the forming die to be formed. Accordingly, the molding sometimes results in variations in the positions of the magnets 2, thereby reducing the accuracy in the positions of the magnets 2. Additionally, since the resin 3 is formed over the outer and inner circumferential faces of the frame 1, the rotor 103 is hard to be decomposed in disposition.
Therefore, an object of the present invention is to provide a rotor for the electric motor whose size, weight and cost can be reduced, which can improve the manufacturing efficiency and can readily be disassembled when disposed of and in which the accuracy in the final positions of the magnets can be improved, and a method of making the rotor.
The present invention provides a rotor for an electric motor including a rotational shaft and a stator having a stator core, comprising a frame previously formed and fixed to the rotational shaft of the motor so as to be rotated with the rotational shaft, and a magnet molded from a molten magnetic resin so as to be disposed at a stator core side of the frame and so as to be integrated with the frame.
According to the above-described rotor, the magnets are made from the magnetic resin so as to be disposed at the stator core side of the frame. Accordingly, since the frame and magnets can be integrated with each other, the resin need not be provided only to integrate the frame and the magnets. Furthermore, components for the magnets need not be disposed at the stator core side of the frame one by one, and the final positions of the magnets are not affected by the result of molding from the resin. Additionally, the resin need not be disposed over the outer and inner circumferential faces of the frame.
In a preferred form, the magnet is anisotropic with respect to poles thereof. Consequently, the magnet can be formed so that the magnetic pole possesses a large magnetic force. In another preferred form, the frame has an axially elongated hole or a plurality of axially aligned holes at a position between magnetic poles of the magnet with respect to the rotational shaft. Consequently, magnetic flux is prevented from leaking through the frame by the elongated hole or axially aligned holes when the magnet is rendered anisotropic. Further, since the magnetic resin fills the elongated hole or axially aligned holes, the strength of the magnet relative to the frame can be improved.
In further another preferred form, the magnet is formed so that a central portion of each magnetic pole has a thickness larger than the other portion thereof. An air gap between each magnetic pole of the magnet is smallest at the central portion of the magnetic pole and largest at both ends of the magnetic pole. Accordingly, the magnetic resistance in the air gap is smallest at the central portion of each magnetic pole and largest at both ends of the magnetic pole. Since the distribution of flux density in the air gap is approximated to a sine wave when regarded as a wave, torque ripple is reduced such that vibration and noise can be reduced.
In further another preferred form, the magnet has an end and a portion opposite the stator and is magnetized so that the end has a larger number of magnetic poles than the portion opposite the stator. Consequently, a rotational position signal can be detected with high accuracy by the magnetic poles of the magnet end.